Previous summer tour partners such as the Doobie Brothers, Huey Lewis & the News and particularly Earth, Wind & Fire were an easy fit for Chicago, which integrated easily with those groups and their music.

The harder-rocking REO Speedwagon, on the other hand, is “a very strange pairing,” says Chicago singer-keyboardist Robert Lamm. But the two groups have found a way to make it work.

Each is joining the other for their respective three-song encores — Chicago coming on for REO’s “Ridin’ the Storm Out,” “Keep on Loving You” and “Roll With the Changes” and REO returning the favor during Chicago’s “Free,” “Does Anybody Really Know What Time It Is?” and “25 or 6 to 4.”

“It’s more like 13,” adds REO frontman Kevin Cronin. “The concept of the two bands together, at least visually and sonically — and certainly volumewise — will give the people more than they plan for.”

Cronin says the idea of “an extended encore ... was really what sold me” to hook up with Chicago.

“For me, the most exciting part of it is that we’re able to cross-pollinate for a while at the end there,” says Cronin, 61. “To hear the Chicago horn section on ‘Roll With the Changes’ is ... very exciting. These guys are, like, real musicians; we’re just a bunch of knuckleheads with guitars. To me as a songwriter, there’s nothing better than when you take a song outside the comfort zone and turn it inside out.”

The two groups also share Illinois roots — Chicago in, duh!, the Windy City, and REO from Champaign. There hasn’t been much contact over the years, though, and Cronin notes that, “It doesn’t make any sense that we haven’t played together.”

“But it’s (also) kind of cool that we’ve never played together, because it makes this tour that much more special for fans of both bands.”